The present invention relates to stripped porous polymer films, as well as methods for preparing such films from substantially heterogeneous porous polymer films.
Porous polymer films are used in a wide variety of applications such as liquid and gas filtration, waterproof breathable fabrics, controlled-release systems, batteries, and fuel cells. Such films may be prepared by a variety of methods. For example, microporous polymer films may be made by extruding a solution of a polyalkene into a film, cooling the resulting film to below the gelling point of the solution, then removing the solvent and stretching the solvent-free film in at least one direction. A process of this kind is disclosed in European Patent Publication No. 0378279 A1, in which a solution of polyethylene in decalin is cooled to below the gelling point, the gel is extruded to form a film from which decalin is removed by evaporation, and the resulting film is then stretched in at least one direction to increase its porosity and mechanical strength.
The applicability of a particular porous polymer film for a given use generally depends upon the structural properties of the film and the properties of its constituent monomers/polymers. For example, known porous polymer films vary in dimensional stability at higher temperatures, thermal/electrical conductivity, mechanical stiffness and mechanical strength, as well as chemical reactivity. Such variation may be due to one or more factors such as, for example, the properties of the constituent monomers/polymers, the effective molecular weight, density, porosity, pore size, thickness, and degree of crosslinking of the film.
Known porous polymer films also may have surface characteristics that are different from those of the interior bulk material. For example, DSM Solutech B.V. manufactures and markets microporous ultra-high molecular weight polyethylene (UHMWPE) membrane materials under the tradename Solupor(copyright) that have higher surface densities and as such have smaller average surface pore sizes relative to the interior bulk material.
Porous polymer films may be useful as substrates for making composite membranes, for example. Particulate solids, polymers that are poor film formers, or polymers that form dense films with mechanical properties that would limit their use in certain applications, can be inserted in pores of a porous polymer film. The resulting composite membranes can have the desired physical properties for use in a wide range of applications. Composite ion exchange membranes suitable for use in electrolytic cells and fuel cells have been described. For example, U.S. Pat. No. 5,834,523 discloses composite membranes comprising porous sheet materials impregnated with xcex1,xcex2,xcex2-trifluorostyrene-based polymeric compositions; suitable microporous films include polyethylene and expanded polytetrafluoroethylene. Composite ion exchange membranes are often preferred in fuel cell applications because they generally have increased mechanical strength in the dry state (which increases ease of handling, for example), and increased dimensional stability (changes in the dimensions of the membrane due to changes in the degree of hydration) in the wet state, compared to dense film ionomeric membranes made by casting or extrusion.
In fuel cell applications, in particular, suitable porous films for incorporation into composite ion exchange membranes preferably have good mechanical and structural properties, a substantially uniform porosity, and are chemically inert. Known porous polymer films can have the requisite mechanical properties, porosity, and chemical inertness. However, the aforementioned difference in the surface characteristics compared to the characteristics of the interior bulk material of some such porous films can be disadvantageous. For example, such heterogeneous films can be resistant to filling with an ion exchange polymer. A polymer solution, for example, brought into contact with the surface of the film may not penetrate the film easily, presumably due to surface density characteristics and/or smaller surface pore size. A more homogeneous porous polymer film having the requisite physical and chemical properties would therefore be advantageous.
A variety of methods may be employed to facilitate penetration of materials into heterogeneous porous polymer films. Surfactant additives have been used to overcome adverse surface interactions with solutions and thereby facilitate penetration, but the presence of surfactants in the final product can be detrimental to subsequent performance of the films, depending upon the application. Furthermore, the removal of surfactants from the final product can be both difficult and costly. The surface density of a heterogeneous porous polymer film could be modified by various means, such as chemical modification (for example, acid etching), heavy atom/particle bombardment, laser ablation, or micro-machining. Such methods can also be costly, however, and may undesirably alter the chemical or structural characteristics of the polymer film surface. Alternatively, the surface layer of a heterogeneous porous polymer film could be removed by high frequency vibrational polishing using a liquid abrasive slurry. However, the process may be difficult to control and/or reproducibility may be difficult to achieve. In any event, the liquid slurry would then need to be removed from the treated porous polymer film, adding an additional step.
A simple and cost-effective method for making a stripped porous polymer film suitable for use as a membrane or substrate in a variety of applications is described herein. The described method involves stripping at least a portion of a surface layer of a heterogeneous porous polymer film, resulting in a more homogeneous porous film.
Also described is a stripped porous polymer film used as a substrate material and composite membranes employing said film as a substrate. Preferably at least a portion of a surface layer of the stripped microporous polymer film has surface characteristics that more closely resemble the characteristics of the interior bulk material. It may also exhibit improved permeability to, and/or penetration of, solids, liquids and gases, increased surface roughness, and increased average surface pore size, relative to the heterogeneous starting material from which it is prepared. Fuel cells, and in particular, membrane electrode assemblies employing such composite membranes, are also disclosed.
In an embodiment of a method for making a stripped porous polymer film from a substantially heterogeneous porous polymer film, the heterogeneous film having surface characteristics that are different from the characteristics of the interior bulk material, the method comprises mechanically stripping at least a portion of at least one surface layer from the heterogeneous porous film. In particular, the method may comprise applying a shearing force to at least a portion of at least one surface layer of the heterogeneous porous polymer film.
The method may comprise attaching a first anchor to a major surface of the heterogeneous porous polymer film, and applying a first shearing force via the first anchor to the major surface to remove at least a portion of a first surface layer from the heterogeneous porous polymer film. The at least a portion of the first surface layer may be associated with the major surface to which the first anchor is attached, or to the opposing surface. In the latter case, the first shearing force removes the surface to which the anchor is attached and the interior bulk material, separating them from the opposing surface.
Optionally, the method may further comprise attaching a second anchor to the opposing major surface of the heterogeneous porous polymer film, and applying a second shearing force via the second anchor to the opposing major surface to remove at least a portion of a second surface layer from the heterogeneous porous polymer film. The first and second shearing forces may be applied simultaneously. Thus, the method may also comprise stripping at least a portion of both surface layers from the heterogeneous porous polymer film, and may further comprise stripping essentially all of both surface layers from the heterogeneous porous polymer film.
Preferably, the shearing forces are applied at a substantially constant angle relative to the plane of the heterogeneous porous polymer film, most preferably at an angle of 90xc2x0 or less from the plane thereof.
The method may be incorporated in a reel-to-reel process in which the heterogeneous porous polymer film is transferred from a feed roller to a collection roller, a first anchor is associated with a first stripping roller, and a first shearing force is applied by the first stripping roller to remove at least a portion of a surface layer of the heterogeneous porous polymer film. The method may include transferring the resultant partially stripped film from a feed roller to a collection roller and removing at least a portion of the other surface layer via the first anchor and first stripping roller. Optionally, the method may incorporate a reel-to-reel process wherein a second anchor is associated with a second stripping roller, and the second stripping roller applies a second shearing force to remove at least a portion of a second surface layer of the heterogeneous porous polymer film. The first and second shearing forces may be applied simultaneously.
Generally, the surface characteristics of the heterogeneous porous polymer film differ from the characteristics of the interior bulk material. Specifically, the surface density of the heterogeneous porous polymer film may be greater than the density of the interior bulk material thereof. The heterogeneous porous polymer film may be microporous. The heterogeneous porous polymer film may comprise a polymer selected from the group consisting of polyethylene, polypropylene, polyvinylidene, polyvinylidene halides, and copolymers thereof. Preferably, it comprises a microporous polymer film comprising a polymer selected from the group consisting of polyethylene, polypropylene, and ethylene-propylene copolymers. More preferably, it comprises a microporous film consisting essentially of polyethylene. Most preferably, it comprises a microporous film consisting essentially of ultra-high molecular weight polyethylene.
A second embodiment is a stripped porous polymer film, wherein the surface characteristics of the stripped film are essentially the same as the characteristics of the interior bulk material, and wherein the stripped film consists essentially of a polyalkene and is preferably made from a precursor film made by a process comprising the steps of:
(a) forming a solution of the polyalkene into a film containing a solvent;
(b) cooling the resulting film to below the gelling point of the solution;
(c) removing the solvent to yield a solvent-free film; and
(d) stretching the solvent-free film in at least one direction.
The stripped porous polymer film, which may be prepared, for example, by the aforementioned methods, may comprise a polyalkene, for example, selected from the group consisting of polyethylene, polypropylene, polyvinylidene, polyvinylidene halides, and copolymers thereof. Preferably, the stripped porous polymer film comprises a polyalkene selected from the group consisting of polyethylene, polypropylene, and ethylene-propylene copolymers. More preferably, it may consist essentially of polyethylene. More preferably, the stripped porous polymer film may consist essentially of ultra-high molecular weight polyethylene.
The stripped porous polymer film may have a surface density lower than the surface density of the heterogeneous porous polymer film from which it is prepared. It may also have a rate of transplanar wicking greater than the rate of transplanar wicking of the precursor heterogeneous film. Further, it may have a Gurley number lower than the Gurley number of the precursor heterogeneous film.
Composite membranes can be made which comprise the present stripped porous polymer film at least partially filled with solid particulate or a liquid composition. For example, metals or metal oxides may be incorporated and used as catalysts. Other materials such as, but not limited to, carbon, glass, or ceramics, may also be employed, depending upon the intended use of the composite membrane.
The stripped porous polymer film in the composite may be at least partially filled with an ion exchange polymer. Optionally, the composite membrane may be at least partially impregnated with a liquid composition of ion exchange polymer. Depending upon the intended application, the resultant composite membrane may be substantially gas impermeable.
Where the composite membrane is an ion exchange membrane, it may be incorporated in a membrane electrode assembly, or in an electrochemical cell, such as a fuel cell